A cross-dresser is a person who wears clothes usually worn by the other sex. The word names the person; the activity is cross-dressing. People cross-dress for many different reasons — for performance, for comfort, for fun, because it feels right, and yes, sometimes for sexual pleasure, which is the reason your dictionary happens to mention. It is worth saying plainly that the dictionary line there is narrower than real use: most of the time the word simply describes what someone wears, and carries no claim at all about why. This matters, because the word is about a real group of people and getting it wrong is unkind. Two things to keep separate. A cross-dresser is not the same as a transgender person: a trans woman wearing a dress is wearing her own clothes, not somebody else's, and calling her a cross-dresser would be a mistake and would hurt. A cross-dresser is also not the same as a drag queen, which is a performer doing an act on a stage. Cross-dresser is the neutral, plain word — it replaced the older term transvestite, which many people now find cold or medical. If you are not sure what someone calls themselves, the safe thing is not to label them at all.